Season the chicken breasts with chili powder, cumin, and salt and pepper and add to the slow cooker. Add the onion, bell pepper, tomatoes, chicken broth, chipotle, and black beans. Add oregano and salt and pepper. Stir, cover with the lid, and cook on high for 4 hours (or on low for 8 hours). After the cooking time is up, shred the chicken in the slow cooker itself or carefully take the chicken breasts out on a chopping board, shred with two forks and return to the slow cooker. Stir in lime juice and check and adjust seasonings.